Fabricating glass-plastic windows

ABSTRACT

A technique for fabricating unbalanced, transparent, glassplastic windows incorporating a technique to compensate for distortion effects similar to the action of a bimetal during the time-temperature-pressure cycle involved in lamination.

United States Patent Inventor Paul E. Shaffer Pittsburgh, Pa.

Appl. No. 32,419

Filed Apr. 27, 1970 Patented Dec. 7, 1971 Assignee PPG Industries, Inc.

Pittsburgh, Pa.

FABRICATING GLASS-PLASTIC WINDOWS 15 Claims, No Drawings U.S. Cl.156/212,

156/99, 156/106, 156/245, 156/309 Iut.Cl B32b 17/06 Field oIScarchl56/2l2,

Primary Examiner-Carl D. Quarfonh Assistant Examiner-Harvey E. BehrendAnomeyChishoim & Spencer ABSTRACT: A technique for fabricatingunbalanced, transparent, glass-plastic windows incorporating a techniqueto compensate for distortion effects similar to the action of a bimetalduring the time-temperature-pressure cycle involved in lamination.

FABRICATING GLASS-PLASTIC WINDOWS BACKGROUND OF THE INVENTION Thisinvention relates to the fabrication of so-called unbalanced,transparent, glass-plastic windows. Glass-plastic laminates are wellknown and have been used as windows for vehicles, buildings andarmor-proof installations. Generally, there are two types ofglass-plastic laminates, the balanced type in-which the laminate hasseveral layers of at least two types of transparent materialsymmetrically arranged with respect to the central layer of the laminateand the unbalanced type in which the layers comprising the laminate arearranged in nonsymmetrical arrangement so that layers of material havingdifferent coefficients of thermal expansion are arranged differently onopposite sides of the planar or curved surface passing through thecenter of the thickness of the laminate.

An assembly of glass and plastic sheets is usually assembled in thefinal arrangement desired for the laminated window to be produced. Theassembled parts are then subjected to an elevated temperature andpressure for sufficient time to adhere the layers together to form atransparent, laminated assembly that has the appearance of a transparentwindow of unlaminated construction. The layers when initially assembleddo not adhere to one another, but adhere when the assembly is at anelevated temperature during the laminating operation. When the layerscool to room temperature after their lamination, each layer tends toretract thermally at a rate dependent on its coefficient of thermalexpansion. In cases where adjacent layers have difierent coefficients ofthermal expansion, stresses are established which tend to distort theshape of the laminated window in a manner similar to the action of abimetal device. In cases of balanced laminates, these stresses tend toequalize so that distortion is not a factor in the case of laminatingbalanced window structures. However, distortion is a factor that becomesvery serious, particularly in unbalanced assemblies comprisingrelatively thick, transparent sheets of polycarbonate resin or acrylicplastic with relatively thin sheets of glass, wherein the thermalexpansion coefficients of the plastic (polycarbonate and/or acrylic) ismany times that of the glass. The distortion resulting from thelamination operation is so great in certain cases that laminates ofglass and transparent sheets of plastic having original shapesconforming to the shape and size of a window opening in whichinstallation is desired prior to their lamination are incapable ofinstallation in the window opening after their lamination.

Window panels for aircraft comprise an outer sheet of glass and aplurality of alternating layers of plasticized polyvinyl butyral whichserve as interlayers and transparent plastic, such as an acrylicplastic, such as methyl methacrylate, and/or a polycarbonate.

Glass is produced by mixing various batch ingredients in a tank andmelting them at a temperature in excess of 2000 F. Sheet glass is drawnupwardly from the tank, whereas plate and float glass is removed betweena pair of forming rolls. The rolled glass is ground and polished in thecase of plate glass and smoothed without grinding and polishing in thecase of float glass by passing the newly formed ribbon in contact over abath of molten tin and controlling the temperature of the molten tinbath along the path of travel taken by the glass ribbon so that thesurfaces of the ribbon are smoothed and the ribbon cooled to atemperature at which its hardened surfaces can withstand contact againsthard surfaces of conveyor rolls that convey the glass ribbon through anannealing lehr.

Glass ribbons, whether of sheet glass, plate glass or float glass, areannealed by controlled cooling through the annealing range of the glassand the strain point. After annealing, the glass ribbon can be cut tothe desired size.

it is preferred that a glass sheet used as the outer sheet of anaircraft window be tempered and preferably the tempering should bechemical in nature, such as provided by the ion exchange technique ofUS. Pat. No. 3,2l8,220 to Neill Weber. It is also preferred that theaircraft window include plasticized polyvinyl butyral and relativelyrigid, transparent plastic sheets such as polycarbonate resin orstretched acrylic plastic. The reasons for these selections follow.

Unlike annealed glass, which usually breaks into a few large,sharp-edged pieces, thermally tempered glass has greater resistance tobreakage on impact, and, when it does break, disintegrates into manysmall fragments, typically cubes having dimensions equal to thethickness of the glass and relatively blunt edges and corners. Both thesize and shape of these fragments make them much less dangerous thanfragments of annealed glass. The average particle size of tempered glassis probably related to the specific strain energy of the glass, i.e.,the elastic energy stored in a unit volume of the prestressed material.This, in turn, depends primarily on the maximum stresses in the glass,since the parabolic shape of the stress distribution in thermallytempered glass is substantially independent of the level of stress.

Strengthening by chemical means, also called chemical tempering, thoughsomewhat newer than the art of thermal tempering, is also well known.There are several mechanisms by which it may be accomplished. One ofthese entails ion exchange in the surface layers of the glass at atemperature approaching the strain point of the glass. In the ionexchange, relatively small ions, such as sodium, are replaced by largerions, such as potassium, or smaller ions, such as lithium, are replacedby larger ions, such as sodium and/or potassium. The crowding of thelarger ions into the spaces left by removal of the smaller ions producesa compression of the surface layers. Two other mechanisms for chemicaltempering entail either ion exchange or partial crystallization, orboth, at elevated temperatures, in such a manner that the modifiedsurface layers of glass have a lower coefficient of expansion than thebase glass. When an article thus treated is cooled to room temperature,the differential contraction of the surface and interior layers againproduces compressive stresses in the surface.

Since diffusion is a relatively slow process, the effects of chemicaltempering do not penetrate very deeply into the glass. This is reflectedin the stress distribution in chemically tempered glass. In such glass,the compressive stress ranges from a relatively high level at thesurfaces to zero at a depth of only a few thousanths of an inch belowthe surface. The rest of the interior of the glass sustains only a verylow tensile stress, required to balance the compressive forces in thevery thin layers near the surfaces. Thus, it may be seen that chemicallytempered glass having the same surface compressive stress as thermallytempered glass may have much less interior tension stress and very muchlower specific strain energy. Thus, while its strength in the absence ofgross surface abrasions may be the same as that of thermally temperedglass, it does not have the same propensity to disintegrate when broken.This may be an advantage in some applications, as will be consideredbelow. lt is usually considered a disadvantage in aircraft applicationswhere the small size of fragments is as important as the enhancement ofstrength. To make for such fine fragmentation, one can raise the surfacecompressive strength or increase the thickness of the compressed layers,or do both, in order to raise the specific strain energy of the glass tothe level at which the particle size of its fragments may be comparablewith that of glass thermally tempered to give a surface compression ofabout 20,000 pounds per square inch. Indeed, the usual chemicallytempered glass intended for aircraft applications is made to havesurface compressive stresses of the order of 80,000 pounds per squareinch.

Comparing thermally and chemically tempered glass, the

former has the advantage that the greater thickness of the compressivelayers on its surface gives it more abrasion resistance. Chemicallytempered glass has the advantage of more readily permitting theattainment of much higher temper stresses, and, therefore, higherstrengths. In addition, it has the advantage that the thickness of thecompressive layer, and with it the specific strain energy of thematerial, may be varied at will, permitting the fracture pattern ofchemically tempered glass to be controlled independently of itsstrength.

It is also well known to chemically temper glass that has beenpreviously thermally tempered. This combination of tempering stepsplaces a higher compression stress in the surface of the glass to alesser depth than the depth of the compression zone produced by thermaltempering, thereby resulting in a stronger glass article than oneproduced by thermal tempering alone.

According to a typical operation, increased impact resistance, breakingstress and penetration resistance are secured in glass by chemicaltempering. In a typical example with an alkali silica glass, forexample, soda-lime-silica glass, a glass sheet is contacted with apotassium salt at a selected temperature range, preferably above 875 F.and below the strain point of the glass, for sufficient time for anexchange to take place in the surface zone of the glass. Preferably, theglass sheet is immersed in a molten bath of a potassium salt, preferablypotassium nitrate. During immersion, an exchange takes place whereinpotassium from the potassium bath is introduced into the glass surface,apparently in exchange for sodium present in the exterior or surfacezone of the glass sheet. It is believed that chemical tempering ofsoda-lime-silica glass is an ion exchange phenomenon wherein potassiumions are exchanged for sodium ions.

Other glass compositions may be chemically tempered by immersion inalkali metal salt baths. For example, an alkali sil ica glass containinglithium may be advantageously chemically tempered by immersion in amolten bath of a sodium salt or a potassium salt or a mixture thereof atan elevated temperature approaching the strain point of the glass. It isalso possible to provide a multiple-step chemical tempering operation inwhich a lithium-containing glass has its lithium ions exchanged forsodium ions, which, subsequently, are exchanged for potassium ions in asecond immersion wherein the sodium-enriched surface zone produced bythe first ion exchange operation becomes a potassium-enriched surfacezone during the second immersion.

After treating the glass composition as recited in the chemicaltempering operations described above, the chemical nature of the alkalimetal oxide constituents of the surface zone of the glass article isaltered radically with replacement of lithium by sodium and/or potassiumor sodium by potassium, depending upon the initial glass composition. Atthe same time the central interior regions of the glass article containsubstantially the same concentration of alkali metal as before thetreatment.

At lower temperatures the effect of such contact with a mo]- ten metalsalt is much slower with the result that chemically tempered glassarticles are difficult to achieve within the periods of time which arecommercially practicable. For example, an immersion of soda-lime-silicaglass for 1 hour in molten potassium nitrate at 700 F. does not improvethe strength properties of the glass substantially. Much longer periodsof immersion at this temperature are required to produce strengthcompressibility to that achieved in the.

minimal time periods (5 to minutes) at higher temperatures. Attemperatures exceeding 870 F., the desired strength improvement occurseven more rapidly.

The upper limit of the contact temperature depends upon the softeningtemperature and melting temperature of the glass article undertreatment. The contact temperature cannot exceed the melting temperatureof the glass composition but it can exceed the strain point and even thesoftening point of the glass composition under certain circumstances.For example, as long as the glass can be supported properly, the contacttemperature can be maintained even at a temperature above the softeningtemperature of the glass provided the contact at these elevatedtemperatures is of sufficiently short duration to avoid thermalrelaxation of the ion-exchange-induced strength characteristics. infact, in some cases it is possible to maintain the contact temperaturewithin the softening temperature range of the particular glass articleundergoing treatment. Under these thermal conditions, extremely shortcontact times can be employed such as on the order of 1 minute or less.

Polyvinyl butyral is formed by reacting butyraldehyde with polyvinylalcohol. The alcohol groups left unreacted are calculated as the percentvinyl alcohol remaining in the polymer. Present-day safety-glasslaminates are made using an interlayer whose base resin is composed of apolyvinyl alcohol partially condensed with butyraldehyde so that itcontains from 15 percent to 30 percent of unreacted hydroxyl groupscalculated as weight percent of vinyl alcohol, less than 3 percent byweight of ester groups calculated as weight percent of vinyl acetate andthe remainder of acetal groups calculated as vinyl butyral. Thismaterial is commonly called polyvinyl butyral or more exactly partialpolyvinyl butyral." conventionally, polyvinyl butyral, as used insafety-glass laminates, contains a plasticizer.

Generally, the plasticizers used are water-insoluble esters of apolybasic acid or a polyhydric alcohol. Particularly desirableplasticizers for use in the present invention are triethylene glycoldi(Z-ethyl-butyrate), dibutyl sebacate, di( beta-butoxyethyl) adipate,and dioctyl phthalate. Other suitable plasticizers include triethyleneglycol fully esterified with a mixture of -90 percent caprylic acid and10-20 percent capric acid as described in US. Pat. No. 2,372,522,dimethyl phthalate, dibutyl phthalate, di(butoxyethyl) sebacate, methylpalmitate, methoxyethyl palmitate, triethylene glycol dibutyrate,triethylene glycol diacetate, tricresyl phosphate, triethyl citrate,butyl butyryl lactate, ethyl para-toluene sulfonamide, dibutyl sulfone,lauryl alcohol, oleyl alcohol, glycerol triricinoleate, methyl lauroylglycolate, butyl octanoyl glycolate and butyl laurate. The above list ofplasticizers does not represent all the known plasticizers which can beused. Such a list would be impractical and would serve no purpose sinceone skilled in the art can readily select a plasticizer from the manyalready known. It has been found preferably to use about 37.5 parts byweight of dibutyl sebacate plasticizer for every parts by weight ofpolyvinyl butyral for aircraft panels with rigid plastic sheets.

The polycarbonate may be any suitable sheet of polycarbonate, such asthat disclosed in US. Pat. Nos. 3,038,365 and 3,1 17,019, and ispreferably prepared by reacting di(monohydroxyaryl) alkanes withderivatives of the carbonic acid such as phosgene and bischloro-carbonicacid esters of di(monohydroxyaryl) alkanes.

The aryl residues of the di(monohydroxyaryl) alkanes can be alike ordifferent. The aryl residues can also carry substituents which are notcapable of reacting in the conversion into polycarbonates, such ashalogen atoms or alkyl groups, for example, the methyl, ethyl, propyl,or tert-butyl group. The alkyl residue of the di(monohydryxyaryl)alkanes linking the two benzene rings can be an open chain or acyeloaliphatic ring and may be substituted, if desired, for example byan aryl residue.

Suitable di(monohydroxyaryl) alkanes are, for example,(4,4'-dihydroxy-diphenyl) methane, 2,2-(4,4'-dihydroxydiphenyl) propane,1,l-(4,4-dihydroxy-diphenyl) cyclohexane,l,1-(4,4-dihydroxy-3,3'-dimethyl-diphenyl) cyclohexane,l,l-(2,2'-dihydroxy-4,4'-dimethyl-diphenyl) butane, (boiling point: -188C. under 0.5 mm. mercury gauge), 2,2(2,24,4'-di-tert-butyl-diphenyl)propane or l, l '-(4,4'-dihydroxy-diphenyl)-l-phenyl ethane;furthermore, methane derivatives which carry besides two hydroxyarylgroups an alkyl residue with at least two carbon atoms and a secondalkyl residue with one or more carbon atoms such as2,2-(4,4'-dihydroxy-diphenyl) butane, 2,2-(4,4-dihydroxydiphenyl)pentane (melting point l49-l50 C.), 3,3-(4,4'- dihydroxy-diphenyl)pentane, 2,2-(4,4'-dihydroxy-diphenyl) hexane,3,3-(4,4'-dihydroxy-diphenyl) hexane, 2,2-(4,4'-dihydroxy-diphenyl)-4-methyl pentane (melting point 15 l-l52C.),2,2-(4,4'-dihydroxy-diphenyl) heptane (boiling point l98-200 C. under0.33 mm. mercury gauge), 4,4-(4,4- dihydroxy-diphenyl) heptane (meltingpoint l48-149 C.), or 2,2-(4,4'-dihydroxy-diphenyl) tridecane. Suitabledi(monohydroxyaryl) alkanes the two aryl residues of which are differentare, for example, 2,2-(4,4-dihydroxy-3'-methyldiphenyl) propane and2,2-(4,4-dihydroxy-3-methyl-3'- isopropyl-diphenyl) butane, Suitabledi(monohydroxyaryl) alkanes the aryl residues of which carry halogenatoms are, for instance 2,2-(3,5,3',5'-tetrachloro-4,4'-dihydroxy-diphenyl) propane, 2,2-(3,5,3,5-tetrabromo-4,4'-dihydroxy-diphenyl) propane,(3,3'-dichloro-4,4-dihydroxy-diphenyl) methane and2,2-dihydroxy-5,5-difluoro-diphenyl methane. Suitabledi(monohydroxyaryl) alkanes the alkyl residue of which linking the twobenzene rings is substituted by an aryl residue are, for instance,(4,4'-dihydroxy-diphenyl) phenyl methane and l,l-(4,4'-dihydroxy-diphenyl l -phenyl ethane.

In order to obtain special properties, mixtures of variousdi(monohydroxyaryl) alkanes can also be used, thus mixed polycarbonatesare obtained.

The conversion of the aforesaid di(monohydroxyaryl) alkanes intohigh-molecular polycarbonates by reacting with the mentioned derivatesof the carbonic acid may be carried out as known in the art. Forinstance, the di(monohydroxyaryl) alkanes can be reesterified withcarbonic acid diesters, e.g., dimethyl-, diethyl-, dipropyl-, dibutyl-,diamyl-, dioctyl-, dicyclohexyl-, diphenyland di-o, p-tolyl carbonate atelevated temperatures from about 50 to about 320 C. and especially fromabout 120 to about 280 C.

The polycarbonates can also be produced by introducing phosgene intosolutions of di(monohydroxyaryl) alkanes in organic bases, such asdimethylaniline, diethylaniline, trimethylamine and pyridine, or intosolutions of di(monohydroxyaryl) alkanes in inert organic solvents, suchas benzine, ligroine, cyclohexane, methylcyclohexane, benzene, toluene,xylene, chloroform, methylene chloride, carbon tetrachloride,trichloroethylene, dichloroethane, methylacetate, and ethylacetate, withthe addition of an acidbinding agent as mentioned above.

A process particularly suitable for producing polycarbonates consists inintroducing phosgene into the aqueous solution or suspension of alkalimetal salts such as lithium-, sodium-, potassiumand calcium salts of thedi(monohydroxyaryl) alkanes, preferably in the presence of an excess ofa base such as lithium-, sodium-, potassiumand calcium hydroxideorcarbonate. The polycarbonate precipitates out from the aqueous solution.

The conversion in the aqueous solution is promoted by the addition ofindifferent solvents of the kind mentioned above which are capable ofdissolving phosgene and eventually the produced polycarbonate.

The phosgene may be used in an equivalent amount. Generally, however, itis preferable to use an excess of phosgene.

Finally, it is also possible to react the di(monohydroxyaryl) alkaneswith about equimolecular amounts of bischloro carbonic acid esters ofdi(monohydroxyaryl) alkanes under corresponding conditions.

In the production of polycarbonates according to the various processes,it is advantageous to employ small amounts of reducing agents, forexample, sodium-, or potassium-sulphide, -sulphite and dithionite orfree phenol and p-tert-butylphenol.

By adding monofunctional compounds which are capable of reacting withphosgene or with the end groups of the polycarbonates consisting of thechlorocarbonic acid ester group and which terminate the chains, such asthe phenols, for instance, the phenol, the tert-butyl-phenol, thecyclohexylphenol, and 2,2-(4-hydroxyphenol-4-methoxy-phenyl) propane,further aniline and methylaniline, it is possible to regulate themolecular weight of the polycarbonates in wide limits.

The reaction of the di(monohydroxyaryl) alkanes with phosgene or of thechlorocarbonic esters of the di(monohydroxyaryl) alkanes may be carriedout at room temperature or at lower or elevated temperatures, that is tosay at temperatures from the freezing point to the boiling point of themixture. (column 1, line 3 l to column 3, line I, of 3,038,365.) Thepolycarbonate film preferably has a thickness of from about 5 to about250 mils and most preferably from about 60 to about 100 mils. In somecases, it may be desirable to use copolymers of various dihydroxy arylpropanes in order to achieve special properties.

Other pellucid materials are disclosed in US. Pat. No. 3,069,301 atcolumn 1, lines 62-68, which are rigid and resistant to scratching andessentially nonhydroscopic.

Acrylic resins, and particularly the polymethacrylates, have found amajor use in the manufacture of aircraft canopies or glazings due inpart to their outstanding optical properties, e.g., clarity andtransparency. Two polymethacrylate resins particularly well adapted forthis purpose are marketed by Rohm & Haas under the trade designations"Plexiglas ll" and Plexiglas 55," both of which resins are essentiallycomprised of polymethylmethacrylate. The two materials are commerciallyavailable in cast sheet form and differ principally in heat resistance,the Plexiglas 55" being the more heat resistant of the two materials.

While the cast acrylic sheeting may be employed in the production ofaircraft glazings, it has been found that the impact strength,resistance to crack propagation, and the craze resistance thereof can besubstantially improved by stretching of the as-cast sheeting. While notattempting to set forth any precise or uncontradictory theory inexplanation of these strength improvements, it is believed that suchstretching affects the molecular structure of the polymer. It appears todisentangle and uncoil the linear molecules and partially orient themparallel to the direction of stretch. This in turn results in theupgrading of the impact strength, resistance to crack propagation andcraze-resistance properties without adversely affecting the excellentoptical properties of the material.

The optimum improvement in the physical properties of an acrylic sheetis obtained when such sheet material is stretched about 75 to percenteither biaxially or multiaxially. Such stretching particularly producesa many-fold increase in resistance to crack propagation. This property,which is a measure of the toughness or shatter resistance of thematerial, is of obvious importance in the glazing of aircraft which arepressurized during flight.

A commonly accepted method for quantitatively expressing the crackpropogation resistance of stretched acrylic materials is the dW/dA valuewhich is a measure of the work absorbed per unit area of crack extensionduring rapid crack growth. As-cast acrylic sheet materialsshow a d W/dAvalue of approximately 4. When stretched 75 to 100 percent, this valueis increased to from 20 to 30. At this level of resistance to crackpropogation, the stretched acrylics show excellent resistance to blowoutfailure when damaged during pressurized flight. As a result of theimproved properties possessed by the stretched acrylics, they have, to aconsiderable extent, replaced the as-cast sheeting for aircraft glazinguse.

It will be appreciated that for certain purposes it is desirable tolaminate the stretched acrylic sheeting either to another sheet ofstretched acrylic, to an as-cast sheet of acrylic, to another plasticmaterial or to glass in producing aircraft glazings. For example, it isdesirable to laminate the stretched acrylic sheet if heating means fordeicing or defogging, such as electrically conducting films or wiregrids, are to be included as part of the aircraft windshield or window.Where a transparent electrically conductive film is used, laminationserves to protect the rather fragile film against damage by scratching,or attack by moisture or corrosive gases in the atmosphere. Also,lamination will provide an insulating cover for the film therebypreventing accidental grounding. Where a conducting wire grid is usedfor deicing or defogging, it can conveniently be imbedded in aninterlayer material which is interposed between a sheet of stretchedacrylic and a second sheet which may be either stretched acrylic or someother synthetic plastic material or glass.

A typical method and apparatus for stretching acrylic plastic isdisclosed and claimed in U.S. Re. Pat. No. 24,978 of Paul H. Bottoms etal.

THE PRESENT INVENTION The present invention suggests a method offabricating an unbalanced, transparent laminated window, comprising atransparent sheet of plastic, a transparent sheet of glass having adifferent coefiicient of thermal expansion from that of said plastic,and an interlayer of bonding material, to a desired ultimate shape,comprising the steps of:

1. bending said glass sheet to a predetermined shape different from saiddesired ultimate shape,

2. bending said transparent sheet of plastic to a shape conforming tosaid predetermined shape and compensating for the combined thickness ofsaid interlayer and said glass sheet,

3. assembling said bent glass sheet and said bent transparent sheet ofplastic to opposite sides of said interlayer of therl moplastic materialto form a window assembly,

4. subjecting said window assembly to an elevated temperature and anelevated pressure for sufficient time to produce a transparent,laminated window, and

5. cooling said laminated window to normal room condi- 2 tions todistort said laminated window into said desired shape.

More particularly, the predetermined shape for the glass sheet isdetermined by laminating an assembly of a glass sheet and a transparentsheet of plastic, both individually bent to the ultimate shape desired,and assembled in opposite order from the arrangement of the ultimateassembly to form a reverse order assembly, and laminated while soassembled in said reverse order with an interlayer under approximatelyidentical conditions of elevated temperature, elevated pressure and timeas those to which said window assembly of said glass sheet bent to saidpredetermined shape, said sheet of plastic bent to said conforming shapeand said interlayer is subjected.

The conforming shape of the sheet of transparent plastic material isreadily determined mathematically by taking into 3 account the totalthickness of the glass sheet and adjacent interlayer. Thus, the exposedsurface of the glass sheet bent to the predetermined shape in thelaminate produced from the reverse order assembly in used as a model fora shaping surface of a mold for bending glass sheets. A second shapingsurface for a conforming shape of a transparent plastic sheet is thendeveloped to determine the shape of the surface of the first transparentplastic sheet adjacent the glass sheet bent to said predetermined shapetaking into account the total thickness of the glass sheet and theinterlayer.

ln fabricating assemblies having more than one sheet of transparentplastic, the shaping surface of each successive mold is calculated fromthe shaping surface of the mold whose surface determines the immediatelypreceding conforming sheet of transparent plastic. The calculation foreach moldshaping surface takes into account the total thickness of thetransparent sheet of plastic and of the intervening interlayer in eachcase.

The following examples of detailed embodiments of the present inventionto laminate flat and curved unbalanced glass-plastic windows willfacilitate an understanding of the present invention.

EXAMPLE I 6 A flat sheet of polymethyl methacrylate sold under the tradename of PLEXlGLAS, 26 inches by 4 inches by A inch, is assembled on oneside of a sheet of interlayer material 0.075 inch thick composed ofpolyvinyl butyral resin plasticized with 37.5i2.0 parts by weight ofdibutyl sebacate per 100 parts of 5 resin and a flat sheet of chemicallytempered glass 0.1 10 inch thick assembled the other side of the sheetof interlayer material. The glass sheet has a chemical compositionconsisting essentially of the following parts by weight: SiO -62.0;

100 F. and maintaining the elevated pressure for an additional 20minutes, the pressure is reduced to atmospheric conditions, the assemblyis removed from the bag and inspected. The assembly, when inspected,possesses a warped shape incorporation a maximum depth of bend of aboutinch with the plastic sheet of the warped assemble having a concavelyshaped outer surface and the glass sheet a convexly shaped outersurface.

EXAMPLE 1! individual glass and plastic sheets having the same outlinedimensions as in example 1 are bent to the outer shapes of the warpedassembly produced as in example 1. An assembly is formed with a plasticinterlayer having the same dimensions as the interlayer in example I,with the bent plastic sheet having its convex surface exposed and theinterlayer against its concave surface and the bent glass sheetassembled with its convex surface against the exposed surface of theinterlayer and its concave surface exposed. The bent assembly isinserted in a bag as in example 1 and subjected to apressure-temperaturetime cycle identical to that described for exampleI. The laminated assembly removed from the bag is flat.

The experiments described in examples I and II demonstrate thefeasibility of the present invention. Similar experiments performed withassemblies similar to those of examples I and 1! except for substitutingpolycarbonate plastic provide the same conclusions.

The utility of the present invention is further illustrated by thesuccessful completion of a test involving bending and laminating acommercial aircraft windshield comprising, in sequence, an outer sheetof chemically tempered glass 0.1 10 inch thick, a first interlayer ofpolyvinyl butyral plasticized with 37.5 parts by weight of dibutylsebacate 0.075 inch thick, a first sheet of stretched polymethylmethacrylate 0.9 inch 5 thick, a second interlayer 0.05 inch thickhaving the same composition as the first interlayer and a second sheetof polymethyl methacrylate 0.7 inch thick. The details of thissuccessful test are described in example lll.

EXAMPLE III A plastic mold having an upper surface conforming to theshape desired for the outer surface of the glass sheet and marking itsoutline and those of the plastic sheets served as a model for makingmolds to shape the glass sheet and the first and second methylmethacrylate sheets forming the window. The glass sheet and each of theplastic sheets were bent to the shape of their respective molds, whichconformed in shape to the mold for the glass sheet, except that the moldfor the first methyl methacrylate sheet has its shaping surfacecompensated for a thickness of 0.185 inch (the total thickness of theglass sheet and the first interlayer) and the mold for the second methylmethacrylate sheet had its shaping surface compensated for a thicknessof 1.585 inches with respect to the glass sheet mold (the totalthickness of the glass sheet, the first methyl methacrylate sheet andthe two interlayers), which corresponds to a compensation of 1.4 incheswith respect to the mold for the first methyl methacrylate sheet (thetotal thickness of the first methyl methacrylate sheet and 0 the secondinterlayer).

The three bent sheets were assembled in the order exactly opposite thatlisted above including alternating interlayers of the aforesaidplasticized polyvinyl butyral of the requisite thicknesses to form areverse order assembly that was inserted within a polyethylene-mylarbag. The bag was evacuated and sealed and the sealed bag and itscontents subjected to a standard laminating cycle that comprisesexposure for 60 minutes to a pressure of 200 pressure per square inch ata temperature of 215 F., then cooling to F. in 20 minutes and holdingthe assembly in the 100 F. atmosphere while continuing to maintain the200 pounds per square inch pressure. The pressure was then removed andthe laminated assembly removed from the bag. It failed to match thecurvature of the glassbending mold which served as a checking fixture.This failure was expected in the light of the teachings in example I and11.

" F. atmospheFe while continuing to The nonconforming laminate soproduced from the reverse order assembly became the model for molds toproduce the final assembly that it was hoped would match the checkingfixture. The mold for the distorted bent glass was designed from theexposed surface of the methyl methacrylate sheet of the nonconforminglaminate produced by laminating the reverse order assembly and the moldsfor the methyl methacrylate sheets were modeled from the mold designatedto conform to the distorted bent glass sheet. However, the mold forproducing the first distorted sheet of plastic compensated for the totalthickness of the glass sheet and the first interlayer, and the mold toshape the second distorted sheet of methyl methacrylate was also similarto the shape of the mold for the distorted glass sheet except that itcompensated for the total thickness of the two plastic interlayers plusthat of the glass sheet plus that of the first sheet of methylmethacrylate.

The sheets of glass and methyl methacrylate were bent to conform to thedistorted shapes of the last three respective molds and assembled in theoriginal order listed, that is, in exactly opposite order to their orderin the reverse order assembly that resulting in the nonconforminglaminate. The correct order assembly of the distorted glass and methylmethacrylate sheets and plasticized polyvinyl butyral interlayer wasthen inserted in a mylar-polyethylene bag. The bag was evacuated andsealed and subjected to the following timetemperature-pressure cycle: 60minutes at 215 F. and 200 pounds per square inch, then cool to 100 F. in20 minutes and hold at l F. for an additional 20 minutes whilemaintaining the pressure at 200 pounds per square inch. The pressure wasthen released and the laminate removed from the bag and the exposedglass sheet held against the checking fixture. This time, the glasssheet conformed in both shape and outline within the tolerances requiredby the customer.

The present invention has taught the glass-plastic laminating art atechnique for laminating unbalanced laminated windows and, particularly,for determining how much distortion to impart to the unplasticizedlayers comprising the unbalanced laminated window. This techniqueinvolved forming a reverse order assembly of parts shaped to the shapedesired in the ultimate laminated window, laminating these parts underconditions identical to the time, temperature and pressure conditionsprevailing during final lamination to form a nonconforming laminate,forming molds conforming to the shapes of certain parts of thenonconforming laminate, shaping said certain parts to said conformingmolds, assembling said certain parts in the proper order separated bythermoplastic interlayers, and laminating said parts conforming to saidconforming molds and said interlayers to produce a laminated windowconforming within desired tolerance to the ultimate shape desired.

While the description above of illustrative embodiments of the presentinvention relates to unbalanced assemblies having interlayers ofplasticized polyvinyl butyral, it is equally applicable to assemblieshaving other well-known interlayer materials, such as polyurethanes,silicones and the like. Similarly, the glass sheet in the laminate maybe of thermally tempered glass or even untempered or partly temperedglass, coated or uncoated, withoutaffecting the manner of practicing theinvention recited in the claimed subject matter that follows.

What is claimed is:

l. A method of fabricating an unbalanced, transparent laminated window,comprising a transparent sheet of plastic, a transparent sheet of glasshaving a different coefficient of thermal expansion from that of saidplastic, and an interlayer of bonding material, to a desired ultimateshape, comprising the steps of;

a. bending said glass sheet to a predetermined shape different from saiddesired ultimate shape,

b. bending said transparent sheet of plastic to a shape conforming tosaid predetermined shape and compensating for the combined thickness ofsaid interlayer and said glass sheet,

0. assembling said bent glass sheet and said bent transparent sheet ofplastic to opposite sides of said interlayer of thermoplastic materialto form a window assembly,

. d. subjecting said window assembly to an elevated temperature and anelevated pressure for. sufficient time to produce a transparent,laminated window, and

e. cooling said laminated window to normal room conditions to distortsaid laminated window into said desired shape.

2. A method as in claim 1, wherein said predetermined shape of saidglass sheet is obtained by duplicating the shape of said glass sheetproduced by laminating an assembly of a glass sheet and a transparentsheet of plastic, both individually bent to the ultimate shape desired,and assembled in opposite order from the arrangement of the ultimateassembly to form a reverse order assembly, and laminated while soassembled in said reverse order with an interlayer under approximatelyidentical conditions of elevated temperature, elevated pressure and timeas those to which said window assemblyof said glass sheet bent to saidpredetermined shape, said sheet of plastic bent to said conforming shapeand said interlayer is subjected.

3. A method as in claim 1, wherein said transparent window comprises aplurality of sheets of transparent plastic with adjacent sheets of saidtransparent plastic separated by an additional interlayer and eachadditional transparent sheet of plastic is bent to a predetermined shapeconforming to the conforming shape of its adjacent sheet of transparentplastic and compensating for the combined thickness of said additionalplastic interlayer and said adjacent sheet of transparent plastic.

4. A method as in claim 3, wherein said predetermined shape of saidglass sheet and said predetermined conforming shapes of each said sheetsof transparent plastic are obtained by duplicating the shapes of theindividual sheets produced by laminating an assembly of a glass sheetand a plurality of transparent sheets of; plastic, all individually bentto the ultimate shape desired and assembled in opposite order from thearrangement of the ultimate window assembly with a layer of interlayermaterial between adjacent sheets to form a reverse order assembly, andlaminated .while so assembled in said reverse order under approximatelyidentical conditions of elevated temperature, elevated pressure and timeas those to which said window assembly comprising said glass sheet bentto said predetermined shape, said sheets of plastic bent to saidpredetermined conforming shapes and said interlayers are subjectedduring said final lamination.

5. A method of producing an unbalanced, curved, glassplastic laminatedwindow comprising a plurality of sheets of transparent material arrangedin asymmetrical relation with respect to the center of the thickness ofthe window comprismg:

a. individually shaping each of a plurality of sheets to conform withthe shape required in said window,

b. assembling said sheets with plastic interlayers separating adjacentof said sheets of transparent material in the reverse order of thatrequired in said window to form a reverse order assembly,

c. laminating said reverse order assembly according to atime-temperature-pressure cycle approximating a predetermined laminatingcycle, thereby forming a laminate that does not conform to the shapedesired for said window,

(1. forming a mold conforming to the shape of each of said sheets ofsaid nonconforming laminate,

e. shaping a sheet to each of the molds formed according to step (d),

f. assembling said sheets shaped as in step (e) with plastic interlayersseparating adjacent of said shaped sheets in the order required for saidwindow to form a proper order assembly,

g, laminating said proper order assembly according to saidtime-temperature-pressure cycle of said predetermined laminating cycle,and

h. cooling said laminated assembly so formed.

6. A method of claim 5, wherein said plastic interlayers are sheets ofplasticized polyvinyl butyral.

7. A method as in claim 5, said plastic interlayers are composed ofpolyurethane.

8. A method as in claim 5, wherein said plastic interlayers are composedof silicone.

9. A method as in claim 5, wherein said laminated window is curved tohave an exposed convex surface and an exposed concave surface, and saidsheet forming said exposed convex surface is glass.

10. A method as in claim 9, wherein said glass sheet is chemicallytempered by ion exchange prior to its assembly.

11. A method as in claim 9, wherein said sheet forming said exposedconcave surface of said laminated window is an acrylic plastic.

12. A method as in claim 11, wherein said glass sheet and said acrylicplastic sheet so treated are of different thickness.

13. A method as in claim 9, wherein said sheet forming said exposedconcave surface of said laminated window is a polycarbonate resin.

14. A method as in claim 13, wherein said glass sheet and saidpolycarbonate sheet are of different thickness.

15. A method as in claim 9, wherein said glass sheet is thermallytempered prior to its assembly.

2. A method as in claim 1, wherein said predetermined shape of said glass sheet is obtained by duplicating the shape of said glass sheet produced by laminating an assembly of a glass sheet and a transparent sheet of plastic, both individually bent to the ultimate shape desired, and assembled in opposite order from the arrangement of the ultimate assembly to form a reverse order assembly, and laminated while so assembled in said reverse order with an interlayer under approximately identical conditions of elevated temperature, elevated pressure and time as those to which said window assembly of said glass sheet bent to said predetermined shape, said sheet of plastic bent to said conforming shape and said interlayer is subjected.
 3. A method as in claim 1, wherein said transparent window comprises a plurality of sheets of transparent plastic with adjacent sheets of said transparent plastic separated by an additional interlayer and each additional transparent sheet of plastic is bent to a predetermined shape conforming to the conforming shape of its adjacent sheet of transparent plastic and compensating for the combined thickness of said additional plastic interlayer and said adjacent sheet of transparent plastic.
 4. A method as in claim 3, wherein said predetermined shape of said glass sheet and said predetermined conforming shapes of each said sheets of transparent plastic are obtained by duplicating the shapes of the individual sheets produced by laminating an assembly of a glass sheet and a plurality of transparent sheets of plastic, all individually bent to the ultimate shape desired and assembled in opposite order from the arrangement of the ultimate window assembly with a layer of interlayer material between adjacent sheets to form a reverse order assembly, and laminated while so assembled in said reverse order under approximately identical conditions of elevated temperature, elevated pressure and time as those to which said window assembly comprising said glass sheet bent to said predetermined shape, said sheets of plastic bent to said predetermined conforming shapes and said interlayers are subjected during said final lamination.
 5. A method of producing an unbalanced, curved, glass-plastic laminated window comprising a plurality of sheets of transparent material arranged in asymmetrical relation with respect to the center of the thickness of the window comprising: a. individually shaping each of a plurality of sheets to conform with the shape required in said window, b. assembling said sheets with plastic interlayers separating adjacent of said sheets of transparent material in the reverse order of that required in said window to form a reverse order assembly, c. laminating said reverse order assembly according to a time-temperature-pressure cycle approximating a predetermined laminating cycle, thereby forming a laminate that does not conform to the shape desired for said window, d. forming a mold conforming to the shape of each of said sheets of said nonconforming laminate, e. shaping a sheet to each of the molds formed according to step (d), f. assembling said sheets shaped as in step (e) with plastic interlayers separating adjacent of said shaped sheets in the order required for said window to form a proper order assembly, g. laminating said proper order assembly according to said time-temperature-pressure cycle of said predetermined laminating cycle, and h. cooling said laminated assembly so formed.
 6. A method of claim 5, wherein said plastic interlayers are sheets of plasticized polyvinyl butyral.
 7. A method as in claim 5, said plastic interlayers are composed of polyurethane.
 8. A method as in claim 5, wherein said plastic interlayers are composed of silicone.
 9. A method as in claim 5, wherein said laminated wIndow is curved to have an exposed convex surface and an exposed concave surface, and said sheet forming said exposed convex surface is glass.
 10. A method as in claim 9, wherein said glass sheet is chemically tempered by ion exchange prior to its assembly.
 11. A method as in claim 9, wherein said sheet forming said exposed concave surface of said laminated window is an acrylic plastic.
 12. A method as in claim 11, wherein said glass sheet and said acrylic plastic sheet so treated are of different thickness.
 13. A method as in claim 9, wherein said sheet forming said exposed concave surface of said laminated window is a polycarbonate resin.
 14. A method as in claim 13, wherein said glass sheet and said polycarbonate sheet are of different thickness.
 15. A method as in claim 9, wherein said glass sheet is thermally tempered prior to its assembly. 